1. Field of the Invention
The invention relates to semiconductor wafer processing, and more specifically, to position control or centering of a wafer on the semiconductor manufacturing tool""s robotic arm, for positioning and repositioning of the wafer during wafer transport.
2. Description of Related Art
As wafers transfer from station to station in a semiconductor manufacturing system, tolerance stack-up as well as handling problems, such as ESC sticking, result in wafer offset that causes process shift, which may ultimately lead to wafer loss. It is therefore desirable for the transfer robot to measure wafer offset and correct this offset at each station transfer.
Typically, multiple sensors are required to empirically and analytically derive the wafer center position. Sensors can be activated upon wafer movement, and a plurality of sensors can be used to determine the wafer""s exact position in a station at a given moment. In U.S. Pat. No. 5,483,138 issued to Shmookler et al. on Jan. 9, 1996, entitled xe2x80x9cSYSTEM AND METHOD FOR AUTOMATED POSITIONING OF A SUBSTRATE IN A PROCESSING CHAMBER,xe2x80x9d a robotic handling system is taught using an array of sensors positioned generally transverse to the path of movement of a substrate to detect the relative positions of the substrate for the purpose of precisely aligning the substrate relative to a predetermined destination point. In Shmookler, a minimum of two sensors is required to practice the disclosed invention.
Other known methods for locating the center point of a semiconductor wafer include those discussed in Spencer et al., U.S. Pat. No. 4,833,790, issued May 30, 1989, entitled xe2x80x9cMETHOD AND SYSTEM FOR LOCATING AND POSITIONING CIRCULAR WORKPIECES,xe2x80x9d and Cheng et al., U.S. Pat. No. 4,819,167, issued Apr. 4, 1989, entitled xe2x80x9cSYSTEM AND METHOD FOR DETECTING THE CENTER OF AN INTEGRATED CIRCUIT WAFER.xe2x80x9d In Spencer, an apparatus, separate and apart from the processing system, utilizes a xe2x80x9cspindlexe2x80x9d type method where the wafer is incrementally rotated. Spencer does not take advantage of the direct movement of the wafer as it is transferred from the wafer storage cassette. Cheng requires an array of optical sensors positioned generally transverse to the linear path of the wafer support blade. Moreover, the wafer must pass over the sensors in a linear path transverse to the position of the sensors, which are positioned adjacent to the loadlock chamber. This makes the Cheng design ill suited to multiple chamber wafer processing systems.
In these processes, multiple sensors are used to locate the wafer in-situ while the wafer is being transferred to or placed within a station. With multiple sensors in an array, each algorithm employed necessarily requires multiple sensor activation information, which is ultimately converted into multiple location coordinate points.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for measuring wafer offset using a single station sensor.
It is another object of the present invention to provide an improved method for determining location of a semiconductor substrate in a wafer processing system and correcting this offset at each station transfer.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention that is directed to a method of centering a wafer within a semiconductor processing apparatus having multiple stations and a single sensor at each of the stations, the method comprising: calibrating a true wafer center position for each of the stations; calculating and storing the calibrated wafer center positions; determining a location for each of the sensors; storing the sensor locations; measuring locations for first and second edge points on the wafer using the single sensor; and calculating a center of the wafer from the stored calibrated wafer position and the first and second edge point locations. The step of calculating the center of the wafer further comprises: analytically deriving intersection points of two circles having the first and second edge points as center points and radii equal to the wafer radius; storing the intersection point closest to the true wafer center position as the center of the wafer; comparing the center of the wafer to the true wafer center position; and adjusting the wafer to the true wafer center position. The method further includes using a paddle having a known feature to locate the true wafer center position, and calculating and storing radial and angular values of the true wafer center for each of the stations. The step of comparing the calculated center of the wafer to the true wafer center position may include matching the true wafer center position with individual transition positions of the wafer, or end effector calibration features, to determine wafer displacement at each of the transition positions. The step of measuring first and second edge points on the wafer may further include activating the sensor a first time by moving the wafer to the station, and activating the sensor a second time by extending the wafer into a module of the station.
In a second aspect, the present invention is directed to a method of centering a wafer having a center and a radius in a semiconductor processing station having a sensor, comprising: calibrating a true wafer center position for the station; activating the sensor at a first transition point; defining the first transition point with a first set of radial and angular coordinates; calculating a second set of radial and angular coordinates for the first transition point; activating the sensor at a second transition point; defining the second transition point with a third set of radial and angular coordinates; and analytically deriving the intersection of two circles having the first and second transition points as center points and radii equal to the wafer radius. The step of analytically deriving the intersection of two circles may include discarding an intersection point furthest from the true wafer center position and storing a remaining intersection point as a calculated wafer center position. The method may further comprise: comparing the calculated wafer center position to the true wafer center position; and adjusting the wafer to the true wafer center position.
In a third aspect, the present invention is directed to a method for detecting a center point of a wafer having a radius and an edge, positioned in a robotic holder of a semiconductor process system using a single sensor for determining the wafer""s instantaneous position, the method comprising: providing one sensor having a defined line-of-sight and capable of activation when the wafer is moved by the robotic holder to cross the sensor""s line of sight; calibrating the semiconductor process system to establish a true center position for the wafer; moving the wafer along a predetermined path to activate the sensor at first and second end points on the wafer""s edge; determining coordinate positions of the true center position, and the first and second end points; calculating a wafer center point from the coordinate positions; and moving the wafer using the robotic holder to adjust the wafer center point to the true center position. The step of calculating a wafer center point further comprises: deriving two circles having centers located at each of the coordinate positions of the end points, and each having a radius equal to the wafer radius; calculating two intersection points of the two circles; and discarding one of the two intersection points furthest from the true center position, using the other of the intersection points as the wafer center point.